Methionine (Met) dependence, the inability of cells to grow when Met is replaced in the culture medium by its immediate precursor homocysteine (Hcy) (Met-Hcy+ medium), is a metabolic defect we have found to be highly prevalent and specific to cancer cells. We have demonstrated that in Met-Hcy+ medium, cells that cannot maintain a high ratio of S-adenosylmethionine (AdoMet) to S-adenosylhomocysteine (AdoHcy) are methionine dependent. Recent results in our laboratory indicate that the overall rate of transmethylation is elevated in human cancer cells compared to normal cells. We postulate that cancer cell types that do not compensate for enhanced transmethylation rates by keeping their AdoMet levels high with respect to AdoHcy become methionine dependent. Besides methionine dependence, other manifestations of elevated rates of transmethylation in cancer cells may be the altered overall DNA and oncogene methylation we have observed in human cancer cells as well as the enhanced rate of excretion of methylated bases and nucleosides observed in cancer patients by a number of laboratories. We have isolated methionine-independent revertants from methionine-dependent human lung cancer, fibrosarcoma, osteogenic sarcoma, and bladder carcinoma cells. These reverted cells have altered karyotypes and altered properties of oncogenic transformation. We shall attempt to determine what products are overmethylated in human cancer cells and to determine if overmethylation is due to overproduction or altered activity of specific methyltransferases. To further understand the relationship of enhanced rates of transmethylation, methionine dependence, altered overall DNA, and oncogenic methylation and oncogenesis itself, we will characterize methionine-independent revertants, isolated from methionine-dependent cancer cells for changes in methionine/transmethylation metabolism, changes in DNA methylation, specifically changes in oncogene methylation, and activity and changes in tumorogenicity. We will also determine the frequency of occurrence of methionine dependence in actual human tumor tissue obtained from surgery by measuring the AdoMet/AdoHcy ratio in Met-Hcy+ medium with the ultimate goal of exploiting this defect to develop a methionine depletion therapy for tumors with the defect of methionine dependence. (N)